This invention relates generally to clutches. More particularly, it relates to a clutch for use in effecting locking engagement between a front drive axle and a front wheel of a four-wheel drive vehicle in response to the application of power to the front drive axle. The clutch automatically effects disengagement upon cessation of the application of power to the front drive axle, together with a direction reversal thereof.
Heretofore, various clutching mechanisms have been used for engaging a front drive axle with its associated wheels in a four-wheel drive vehicle. One such mechanism normally is disengaged to allow the wheels to rotate independently of the front drive system. This requires that the operator lock each clutch manually to engage the front drive axle and wheels, and to unlock them manually to disengage.
Another such mechansim provides an overrunning clutch which engages automatically when power is applied to the front drive axle and when operation is in the drive mode. However, such an overrunning clutch disengages automatically upon operation in the coast mode. In other words, the overrunning clutch engages when the rotational speed of the axle tends to exceed the rotational speed of the wheel, but disengages when the rotational speed of the wheel tends to exceed that of the axle. Such overrunning clutches generally provide some means by which the operator may override manually to insure locking engagement between the axle and wheels.
Yet another such mechanism provides a clutch which operates in response to the application of torque to the front drive axle to move pins into slots so as to engage the axle with its associated wheels. Although a mechanism of this type will effect engagement in either the drive or coast mode of operation, there is the possibility that the pins will slip out of the slots during movement between drive and coast, in which case the clutch would disengage and then re-engage automatically. At normal operating speeds such disengagement and re-engagement could cause severe shocks to the clutch components and, indeed, to the entire front driveline. This would result in a dangerous and possibly destructive condition. Further, in a float condition wherein the axle is rotating but no torque is transferred between the axle and wheels, an inadvertent tendency for movement between the drive and coast modes of operation could develop. This could cause the clutch to disengage and then re-engage, and establish the same dangerous condition.
U.S. Application Ser. No. 799,793 filed May 23, 1977 is directed to an automatic locking clutch which overcomes the deficiencies of the prior devices. As disclosed therein, the clutch will automatically engage a front drive axle and an associated wheel in response to engagement of the front-wheel drive system, will maintain engagement positively in the drive and coast modes of operation as well as during the transition between drive and coast, will maintain engagement positively in forward and reverse operation as well as during the transition between forward and reverse, and will disengage automatically when the front-drive system is disengaged.
The automatic locking clutch includes a ring for developing frictional drag sufficiently high to cause clutch engagement. Thereafter, this high drag continues, although it is not required to maintain engagement. Thus, in order to conserve power, reduce heat, ease the problems of material selection, etc., there remains a need for an automatic locking clutch of this type which will develop relatively high frictional drag initially to effect automatic clutch engagement, and which will develop relatively low frictional drag thereafter.